Torque Converter?

[ticopowell]

Ok so i have seen a lot of posts mentioning the Torque converter, but I am not exactly sure what it is, nor what it does. I know that it will lock and unlock depending on what gear you are in, and what your speed is, and how hard you are pushing the engine, but other than that all i can tell from reading is that my truck has it. I was looking at the gear selection display thingy on my gryphon and I noticed that sometimes, when driving steadily in 4th gear down the freeway, a lock symbol would appear. I assume this is the torque converter locking. So yeah any explanation would be great